The invention generally relates to design, fabrication and application of optical superlattice with special functions in optical parametric process. The optical superlattice can be used as an efficient frequency-conversion crystal to produce short-wavelength, multi-wavelength and tunable wavelength laser in the all-solid state laser.
With the development of modern advanced technique, lasers at various wavelength ranges are in practical need. In some wavelength ranges and at some wavelength, current semi-conductor laser or solid-state laser cannot be utilized or utilized effectively. For example, there are potential or practical needs for red, green and blue three fundamental colors all solid-state laser for laser display, blue-green laser for under-water communication, broad-band tunable laser for radar, remote sensing and environment supervising, and etc. In many application fields, mini-type (average power from tens of mini watts (mW) to several watts (W)), all solid-state laser with a couple of nonlinear crystals as frequency conversion medium can offer some wavelengths and wavelength ranges extending beyond the range that can be generated with diode laser.
An all solid-state laser usually use an efficient GaAlAs diode laser (LD) as a pump source whose emitting wavelength close 800 nm to pump a crystal doped with a laser active ion. For example, the neodymium (Nd) can cause lasing at various wavelengths in the near infrared (the accurate wavelengths are related to their host crystal), Nd doped yttrium aluminum garnet (YAG) is the most popular host. The emitting of laser crystal can be converted to other wavelength via nonlinear effect such as in a homogenous nonlinear optical crystal by birefringence-phase-matching (BPM)) or in an optical superlattice, whose nonlinear coefficient is modulated, by quasi-phase-matching (QPM).
The emitting wavelengths and other parameters of several Nd doped laser crystals are listed in Table 1.
Table 1 shows that there are three groups of emitting lines suitable for application in some ordinary Nd doped crystals: the first is around 900 nm; the second is around 1064 nm; the third is around 1300 nm. For example, The most efficient emitting line for the most commonly used Nd:YVO4 crystal occurs at 914 nm, 1064 nm and 1342 nm, respectively. In addition to Nd, several other active ions like Er, Yb, Ho, and so on can be doped into various crystals to produce emitting at other wavelengths. In order to obtain shorter wavelength laser (e.g. red, green, blue and ultraviolet) or longer wavelength laser (e.g., mid-infrared), one needs to make use of nonlinear optical effect in nonlinear crystal such as KTP or an optical superlattice such as PPLN to convert above emissions into novel wavelengths. For example, by QPM frequency doubling these three lines, 1342 nm, 1064 nm and 914 nm, of Nd:YVO4, using three pieces of periodic optical superlattices with different modulated periods, respectively, one can obtain red at 671 nm, green at 532 nm and blue at 457 nm, respectively. And also the emitting lines mentioned above can be used as pump sources to produce radiations at other wavelengths by optical parametric processes.
Recently, optical superlattice (alternatively named quasi-phase-matching (QPM) material) is more and more used as a frequency conversion crystal in all-solid state laser, and eventually steps into application. The theoretical basis of optical superlattice used for laser frequency conversion is QPM scheme first proposed by Bloembergen et al. in 1962. The theory can be described briefly like this: in the optical parametric process, phase mismatch between different parametric waves due to refractive index dispersion can be periodically compensated in a medium with periodically modulated nonlinear optical coefficient, which can efficiently realize the energy conversion among parametric waves. In comparison with birefringence-phase-matching, much attention has been paid to QPM because of its merits: higher efficient nonlinear coefficient, realization of phase-matching in whole transparent region of crystal, no walk-off angle, and so on. D. Feng, N. B. Ming, et al. in 1980 made use of growth striation technique to yield a single crystal of LiNbO3 with periodic ferroelectric domains (PPLN, or called optical superlattice), where the nonlinear optical coefficient was modulated periodically through the ferroelectric domains. With the crystal, it was the first time for them to acquire the efficient SHG from a 1064 nm Nd.YAG laser by QPM scheme. Yamada et al. of Sony in Japan realized the periodical inversion of ferroelectric domains in z-cut LiNbO3 crystal wafer with pulse electric field poling technique firstly at room temperature, and then acquired efficient blue by QPM SHG in 1993. Yamada et al. used metal electrodes and multi-pulse technique. Whereafter Byer et al. used liquid electrodes; S. N. Zhu et al. and Rosenman et al. used single-pulse technique realizing periodic arrangement of ferroelectric domains in LiNbO3, LiTaO3, and KTiOPO4, and acquiring efficient SHG of blue within these crystals, respectively. Up to date, this kind of superlattice materials have been applied extensively in various kinds of optical parametric processes, such as SHG, sum-frequency generation (SFG), optical parametric oscillation and amplification (OPO/OPA), and so on, where laser can be converted from some fixed frequency into a novel frequency or a couple of novel frequencies or can be tunable in some frequency range, which meets the demands for different wavelength in various application fields.
Y. Y. Zhu, N. B. Ming et al. firstly introduced Fibonacci structure into optical superlattice in 1990. S. N. Zhu, Y. Y. Zhu and N. B. Ming published xe2x80x9cQuasi-Phase-Matched Third-Harmonic Generation in a Quasi-Periodic Optical Superlatticexe2x80x9d on SCIENCE in 1997. In this work, they firstly proposed the thought of directly acquiring third-harmonic generation (THG) through the simultaneous realization of QPM in SHG and sum-frequency generation processes in a single superlattice, which has been verified experimentally. They adopted LiTaO3 superlattice with Fibonacci domain sequence as nonlinear crystal, acquiring green light at 523 nm which is a third harmonic of fundamental light at 1570 nm. The work verified QPM multi-parametric processes can be simultaneously realized, and be coupled mutually by cascade effect to produce novel wavelength. However, Fibonacci sequence can only realize such a process with the fundamental light at that given wavelength above, it doesn""t fit any other wavelength, in particular those available wavelengths, such as the corresponding wavelength emitting from those doped laser crystals or semi-conductor lasers introduced above.
A laser system in accordance with the principles of the present invention includes an active ion doped laser crystal. A pumping source is in communication with the laser crystal so as to pump the laser crystal. A resonant cavity with an output mirror is in communication with the laser crystal so as to resonate light from the laser crystal.
A frequency conversion crystal, disposed within a heater, is located along the optical path of the laser, either inside or outside of the resonant cavity. The frequency conversion crystal is an optical superlattice of quasi-phase-matching material, and is adapted to simultaneously perform two or more parametric processes for frequency conversion of light from the laser.
This invention aims at the design of optical superlattice based on three groups of available emitting lines (900 nm, 1064 nm and 1300 nm) of Nd3+ doped crystals to realize short-wavelength lasers (like blue and ultraviolet), multicolor lasers (like red-blue, green-ultraviolet and red-green-blue) or the output of tunable lasers by QPM third-harmonic generation or other coupled optical parametric processes. These optical superlattices may be designed with different structures (including periodic, quasi-periodic, dual-periodic, aperiodic and etc.) that dependent on QPM conditions and expectant efficiency. Therefore, the optical superlattice can serve as nonlinear optical crystal to construct a LD(Laser Diode) pumped all-solid state laser to generate lasers in some particular wavelengths in visible, infrared and ultraviolet or to generate tunable laser in some wavelength range.